Many search engine services, such as Google, Yahoo! and MSN, provide for searching for information that is accessible via the Internet. These search engine services allow users to search for web pages and other Internet-accessible resources that may be of interest to users. After a user submits a search request that includes search terms, the search engine service identifies web pages that may be related to those search terms. To quickly identify related web pages, the search engine services may maintain a mapping of keywords to web pages. This mapping may be generated by “crawling” the web (i.e., the World Wide Web) to identify the keywords of each web page. To crawl the web, a search engine service may use a list of root web pages to identify all web pages that are accessible through those root web pages. The keywords of any particular web page can be identified using various well-known information retrieval techniques, such as identifying the words of a headline, the words supplied in the metadata of the web page, the words that are highlighted, and so on. Some search engine services can even search information sources that are not accessible via the Internet. For example, a book publisher may make the content of its books available to a search engine service. The search engine may generate a mapping between the keywords and books. When a search engine service receives a search request that includes one or more search terms, it uses its mapping to identify those information sources (e.g., web pages or books) whose keywords most closely match the search terms. The collection of information sources that most closely matches the search terms is referred to as the “search result.” The search engine service then ranks the information sources of the search result based on the closeness of each match, web page popularity (e.g., Google's page ranking), and so on. The search engine service then displays to the user links to those information sources in an order that is based on their rankings.
Some search engine services do not charge a fee to the providers of web pages for including links to their web pages in search results. Rather, the search engine services obtain revenue by placing advertisements along with search results. These paid-for advertisements are commonly referred to as “sponsored links,” “sponsored matches,” or “paid-for search results.” A vendor who wants to place an advertisement along with certain search results provides a search engine service with an advertisement and search terms. When a search request is received, the search engine service identifies the advertisements whose search terms most closely match those of the search request. The search engine service then may display some of the advertisements along with the search results. If more advertisements are identified than will fit on the first page of the search results, the search engine service may select to display on the first page those advertisements based on some criterion such as bid amount. In general, a search engine service or other advertisement placement service will display advertisements with higher bid amounts more prominently. For example, a list of sponsored links may be ordered by bid amount, the advertisement with the highest bid amount may be displayed in a prominent location (e.g., top of a web page), an advertisement with a high bid amount may be highlighted, and so on. The search engine services can either charge for placement of each advertisement along with search results (i.e., cost per impression) or charge only when a user actually selects a link associated with an advertisement (i.e., cost per click).
Many web sites receive referrals from other web sites. Such referrals can be a source of revenue for providers of both the referring web site and the referred web site. For example, a medical information web site that provides medical information may include links to refer users to a bookseller's web site through which users can purchase books. A link may refer a user to a detailed web page for a specific book, to a browse node web page through which the user can browse through lists of medical books, or to a search web page that provides search results for a query included in the link. The referred web site may track the activity of the referred user while visiting the web site. If the user makes a purchase during the visit (or session), the provider of the referred web site may compensate the provider of the referring web site for the referral. Thus, the compensation serves an incentive for the referring web site to continue referring users to the referred web site. The referred web site provider benefits from the referral since it increases its chances of selling its products. The referred web site may maintain a record of each referral in order to properly compensate the provider of the referring web sites and to assess the benefit of the referral program. Not all “referrals,” however, need be compensated. For example, a search engine service may provide a link (i.e., not a sponsored link) to a web site as part of its search result. In such a case, the provider of the web site would not typically compensate the provider of the search engine service.
In addition to receiving referrals, a web site provider may also place advertisements for the web site with order placement services such as search engine services. As an advertiser, the provider of a web site would like to maximize the effectiveness of advertising dollars used to pay for advertisements. Thus, advertisers try to identify keyword and advertisement combinations that result in the highest benefits (e.g., most profit) to the advertiser. Many techniques have been developed to identify keywords that may be appropriate for advertising various items. For example, some techniques analyze “clickthrough logs” to identify keywords of search requests submitted by users and the items of sponsored links that the users selected. If many search requests with a common keyword result in users selecting sponsored links for the same item, then a vendor may want to place an advertisement for that item or for the category (e.g., DVDs and books) of that item with results of search requests that contain that search term.
Some techniques also select search terms based on a “conversion rate” for a search term and an item. A conversion rate is a measure of the percentage of clickthroughs to the item resulting in an actual purchase of an item. Conversion rate, however, is more generally the percentage of clickthroughs that result in some desirable benefit to a vendor or an organization. For example, the conversion rate for an insurance company may be a measure of the percentage of clickthroughs that result in the user requesting a rate quote.
Because clickthrough logs collected over months and years provide a detailed and extensive picture of user activity relating to search terms, the techniques that mine the clickthrough logs for keyword and item combinations or keyword and category of items combination for advertisement purposes have been effective. These techniques, however, tend to be slow to react to rapid changes in user activity. For example, when a new medical breakthrough is announced, an advertiser may not have attempted to place advertisements based on keywords appropriate for the medical breakthrough. Indeed, it may take a considerable amount of time for the advertiser to detect user interest in the breakthrough and place related advertisements. Many special-purpose web sites, however, may experience a rapid change in user activity due to recent events (e.g., an announced visit of a dignitary). For example, users may visit a medical information web site to obtain more information about the medical breakthrough. Because the medical information web site focuses solely on medical information, it may experience a rapid increase in traffic related to the breakthrough after the announcement. The medical information web site may refer its users to a bookseller's web site using a query relating to the breakthrough. For example, if the breakthrough relates to a cure for breast cancer, users may submit queries such as “breast cancer cure Mayo clinic.” When the bookseller's web site receives the referral, it searches for books that match the query and provides to the user a web page with the books listed. As described above, however, the bookseller's reliance on analysis of clickthrough logs will not typically be able to quickly detect the increased user activity.